Printed wiring boards are well known in the electronics industry. Such boards typically consist of a thermosetting resin matrix reinforced with a fibrous material. The fibrous reinforcing material is normally glass, although other dielectric reinforcing materials such as paper, quartz and aramid have also been used. The printed wiring board is completed by the creation of appropriate patterns of electrically conductive material on one or both surfaces of the board.
Two important characteristics of a printed wiring board are its coefficient of thermal expansion and its thermal conductivity. Differences in the coefficient of thermal expansion between a printed wiring board and the components, solder and plating on the board can result in solder joint cracking and failure when the assembly is subjected to temperature variations. If components such as ceramic chip carriers are to be mounted to the board, it is important to match the coefficient of thermal expansion in the plane of the board (X-Y direction) to the coefficient of thermal expansion of the ceramic chip carriers, while retaining a coefficient of thermal expansion in the thickness (Z) direction approximating that of solder, plating and component leads.
The coefficient of thermal expansion of a conventional glass fiber reinforced epoxy printed wiring board is approximately 14-17 parts per million/degree Celsius (ppm/.degree.C.). This value is considered acceptable for attachment of axial leaded, radial leaded, DIP and flatpack components to the board. Ceramic chip carriers, however, have coefficient of thermal expansion of 4.8 to 6.5 ppm/.degree.C. One known technique of lowering the X-Y coefficient of thermal expansion of glass epoxy boards has been to employ laminates, such as those disclosed in U.S. Pat. No. 4,318,954. In the laminates disclosed in that patent, the printed wiring board has a support composed of graphite fiber reinforced resin. Because graphite fibers have a negative coefficient of thermal expansion, the X-Y coefficient of thermal expansion of the support can be made very small (less than 3.6 ppm/.degree.C.), resulting in an apparent X-Y coefficient of thermal expansion of the resultant printed wiring board approximating that of a ceramic chip carrier.
As mentioned above, a second important characteristic of a printed wiring board is the board's thermal conductivity. In many applications, convective air cooling of electronic components is not possible or practical, and alternative methods of conveying heat away from electronic components are required. It would therefore be desirable to produce a printed wiring board having a thermal conductivity high enough to transfer large heat loads. The high thermal conductivity of the board, however, would have to be achieved without sacrificing other important properties, such as an appropriate coefficient of thermal expansion. To date, no glass-epoxy based printed wiring boards have been available that combine high thermal conductivity with a low coefficient of thermal expansion.